Intrachain charge transfer complexes

ABSTRACT

Method for achieving free radical initiated copolymerization of an addition monomer having pendant therefrom a strong donor group with an addition monomer having pendant therefrom a strong acceptor group. Copolymers prepared according to this method can be represented by the following formula: ##STR1## wherein R is hydrogen or methyl; 
     R&#39; is hydrogen or methyl; 
     R&#34; is hydrogen or methyl; 
     R&#39;&#34; is selected from the group consisting of --NO 2 , halogen, --CN and --CF 3  ; 
     X and Y are independently selected from the group consisting of hydrogen, chlorine, bromine, alkyl of 1-4 carbon atoms and phenyl; 
     a and b can range from 0-4; and 
     n and m can range from about 5 to about 95 percent. 
     The polymeric compositions prepared according to this method are suitable for use in electrophotography either alone as the primary photoresponsive entity or in combination with other photoconductive materials.

This is a division of application Ser. No. 596,531, filed July 16, 1975.

FIELD OF THE INVENTION

This invention relates to a method for preparation of polymericcompositions and the use of these compositions in electrophotographicelements and processes. More specifically, this invention involvesrandom copolymers suitable for use in electrophotographic imagingmembers and processes. The spacial constraint, relative conformation,relative ionization potential and relative electron affinity of thependant groups of the two principal components of these compositionsfavors charge transfer interaction between them, resulting in highlycolored polymeric materials.

DESCRIPTION OF THE PRIOR ART

The formation and development of images on the imaging surfaces ofphotoconductive materials by electrostatic means is well known. The bestknown of the commercial processes, more commonly known as xerography,involves forming a latent electrostatic image on an imaging surface ofan imaging member by first uniformly electrostatically charging thesurface of the imaging layer in the dark and then exposing thiselectrostatically charged surface to a light and shadow image. Thelight-struck areas of the imaging layer are thus rendered relativelyconductive and the electrostatic charge selectively dissipated in theseirradiated areas. After the photoconductor is exposed, the latentelectrostatic image on this image-bearing surface is rendered visible bydevelopment with a finely divided colored marking material known in theart as "toner". This toner will be principally attracted to those areason the image-bearing surface having a polarity of charge opposite to thecharge on the toner particles, thus forming a visible powder image.

The developed image can then be read or permanently affixed to thephotoconductor where the imaging layer is not to be reused. This latterpractice is usually followed with respect to the binder-typephotoconductive films (e.g. ZnO dispersed in a resinous binder) wherethe photoconductive imaging layer is also an integral part of thefinished copy.

In so-called "plain paper" copying systems, the latent image can bedeveloped on the imaging surface of a reusable photoconductor ortransferred to another surface, such as a sheet of paper, and thereafterdeveloped. When the latent image is developed on the imaging surface ofa reusable photoconductor, it is subseqently transferred to anothersubstrate and then permanently affixed thereto. Any one of a variety ofwell known techniques can be used to permanently affix the toner imageto the copy sheet, including overcoating with transparent films, andsolvent or thermal fusion of the toner particles to the supportivesubstrate.

In the above "plain paper" copying system, the materials used in thephotoconductive layer should preferably be capable of rapid switchingfrom insulating to conductive to insulating state in order to permitcyclic use of the imaging surface. The failure of a material to returnto its relatively insulating state prior to the succeeding chargingsequence will result in a decrease in the maximum charge acceptance ofthe photoconductor. This phenomenon, commonly referred to in the art as"fatigue", has in the past been avoided by the selection ofphotoconductive materials possessing rapid switching capacity. Typicalof the materials suitable for use in such a rapidly cycling systeminclude anthracene, sulfur, selenium and mixtures thereof (U.S. Pat. No.2,297,691); selenium being preferred because of its superiorphotosensitivity.

In addition to anthracene, other organic photoconductive materials, mostnotably, poly(N-vinylcarbazole), have been the focus of increasinginterest in electrophotography. Most organic photoconductive materials,however, including poly(N-vinylcarbazole), lack the inherentphotosensitivity to be competitive with selenium. This need for theenhancement of the photoresponse characteristics of organicphotoconductors thus led to the formulation of these organic materialswith other compounds, commonly referred to as "activators".Poly(vinylcarbazoles), for example, when sensitized with2,4,7-trinitro-9-fluorenone exhibit good photoresponse and dischargecharacteristics and, (depending upon the polarity of the surfacecharge), low dark decay; U.S. Pat. No. 3,484,237. Other organic resins,traditionally considered nonphotoconductive can also be sensitized withcertain activators, such as Lewis Acids, thus forming charge transfercomplexes which are photoresponsive in the visible band of the spectrum,U.S. Pat. Nos. 3,408,181; 3,408,182; 3,408,183; 3,408,184; 3,408,185;3,408,186; 3,408,187; 3,408,188; 3,408,189; and 3,408,190. With respectto both the photoconductive and nonphotoconductive resins, the degree ofsensitization is generally concentration dependent; the higher theloadings of activators, the greater the photoresponse.

The concentration of activator capable of formulation with the abovematerials, however, is finite; generally being limited to less than 10weight percent of the composition. Ordinarily, the addition of highloadings of activator to many of the above materials will lead toimpairment of mechanical and/or the photoconductive properties of thesensitized composition. In most instances, the excessive addition ofactivators to both the photoconductive and nonphotoconductive materialsof the types disclosed in the above patents will result incrystallization of these activators, thus impairing the mechanicalstrength and other physical properties of the resultant photoconductivecomposition. Still yet other sensitizers, when present in relatively lowconcentration can result in oversensitization of the composition in thatthe photocurrent generated upon exposure will persist long afterillumination ceases, BUL. CHEM. SOC. of JAP. 39, 1660 (1966). Thisphenomenon prevents the further use of such materials for preparation ofsuccessive electrostatic reproductions until such persistentconductivity is dissipated in the previously illuminated areas of thephotoconductor. The dissipation of persistent photocurrents generallytakes an extended period of time and/or thermal erasure, thus makingthese oversensitized compositions generally unsatisfactory for rapidcycling electrostatographic imaging systems.

As an alternative to the more traditional type of sensitizationdiscussed above, Inami and Morimoto have proposed preparation of"intramolecular" charge transfer complexes (more properly characterizedas "intrachain" charge transfer complexes) wherein electron donor andelectron acceptor substituents are located along a common vinylbackbone, U.S. Pat. No. 3,418,116. The materials of principal interestdisclosed in the above patent are the nitrated polymers ofpolyacenaphthylene, poly-9-vinylcarbazole and poly-1-vinylnaphthalene.Intrachain charge transfer complexes have also been disclosed byPodhajny (U.S. Pat. No. 3,697,264) and Limburg (U.S. Pat. No.3,877,936). All of the intrachain charge transfer complexes disclosed todate comprise relatively strong electron donor structural units andrelatively weak electron acceptor structural units. Attempts atpreparation of monomers having strong electron acceptor groups (groupshaving an electron affinity in excess of about 0.7 electron volts) haveup to now been generally unsuccessful. Even in the limited instanceswhere it has been possible to prepare such monomers, polymerization ofthese monomers by free radical initiation has been virtually impossible,since the electron acceptor moiety quenches the free radical.Introduction of strong electron acceptor substituents onto preformedpolymer backbones has also encountered considerable difficulty. Forexample, attempts at nitration of poly(vinylfluroenone) results indegradation in the polymer chain and reduction in its solubility incommon solvents (presumably due to cross-linking).

Accordingly, it is the object of this invention to remove the above aswell as related deficiencies in the prior art.

More specifically, it is the object of this invention to provide amethod for enhancement of electron donor and electron acceptorinteraction so as to increase the probability of charge transfer complexformation.

It is primary object of this invention to provide a method and means forachieving free radical initated copolymerization of an addition monomerhaving pendant therefrom strong electron donor groups with an additionmonomer having pendant therefrom strong electron acceptor groups.

It is another object of this invention to provide a polymericcomposition capable of formation of an intrachain charge transfercomplex.

It is yet another object of this invention to provide a copolymercomposition having a random distribution of relatively strong electronacceptor and relatively strong electron donor structural units.

Additional objects of this invention include dilution of the aboverandom copolymer so as to minimize charge transfer interaction and yetmaintain the ambipolar transport properties of the composition.

SUMMARY OF THE INVENTION

The above and related objects are achieved by providing a method forperforming free radical initiated copolymerization of an additionmonomer having pendant therefrom a strong electron donor group with anaddition monomer having pendant therefrom a strong electron acceptorgroup. According to this method, acrylate or methacrylate substituentsare formed on a strong electron donor group and a strong electronacceptor group respectively, the acrylate or methacrylate monomers thusprepared combined in a suitable solvent and their copolymerizationinitiated by the free radicals. Copolymers prepared in the mannerdescribed above are highly colored due to the formation of an intrachaincharge transfer complex between the strong donor and strong acceptorgroups pendant from the compolymer backbone. These copolymers arerepresented by the following formula: ##STR2## wherein R is hydrogen ormethyl;

R' is hydrogen or methyl;

R" is hydrogen or methyl;

R'" is selected from the group consisting of --NO₂, halogen, --CN and--CF₃ ;

X and Y are independently selected from the group consisting ofhydrogen, chlorine, bromine, alkyl of 1-4 carbon atoms and phenyl;

a and b can range from 0-4; and

n can range from about 5 to about 95 percent of the total number ofstructural units of the copolymer; and

m can range from about 5 to about 95 percent of the total number ofstructural units of the copolymer.

DESCRIPTION OF THE INVENTION INCLUDING PREFERRED EMBODIMENTS

The copolymers of this invention are prepared by the standard freeradical initiated copolymerization of an anthracenic functional monomerwith a 9-fluorenyl functional monomer. Anthracenic functional monomerssuitable for use in preparation of these copolymers are defined by thefollowing formula: ##STR3## wherein R, R', X and Y are as previouslydefined.

These anthracenic functional monomers can be prepared by acylation ofanthracene or a substituted anthracene, at the 2-position followed byreduction of the acylated anthracene to the corresponding alcohol. Thisalcohol can thereafter be condensed with an acryloyl halide or analpha-alkylacryloyl halide, thereby forming a monomer having thestructure set forth hereinabove.

The 9-fluorenyl or substituted 9-fluorenyl monomers suitable for use inpreparation of copolymers of this invention have the following formula:##STR4## wherein R", R'", a and b are as previously defined.

These monomers can be prepared by reacting a 9-hydrazone derivative offluorenone or a 9-hydrazone derivative of a substituted fluorenone underoxidizing conditions, thereby converting the hydrazone to thecorresponding 9-diazo compound. This 9-diazo compound is subsequentlycontacted with an alpha alkyl substituted acrylic acid and theesterification of these two ingredients catalyzed by the introduction ofa Lewis acid.

After the individual monomers have been prepared in the manner describedabove, they can be combined in a suitable solvent, such as acetone, anda conventional free radical initiator subsequently added to the monomercharge. The copolymerization of these materials can thereafter procedein the conventional manner. The copolymeric product which is producedfrom these materials can be precipitated in an alcohol, such asmethanol, and thereafter purified in the conventional manner. Analysisof the copolymer products produced in the manner described aboveindicates that the concentration of donor and acceptor units in thecopolymer corresponds to the original concentration of the monomers inthe charge prior to polymerization. The sequence distribution of thestructural units in the copolymer appears to be random.

The copolymers prepared in the manner described above are highly coloredand, therefore, sensitive to light in the visible region of theelectromagnetic spectrum. These copolymers can be used asphotoconductors in electrophotographic imaging members and methods. Theintensity of charge transfer interaction between the electron donor andelectron acceptor groups of this copolymer can be reduced and thus thecolor of the copolymer by simple dilution of the frequency of chargetransfer interaction. This achieved by introduction of a third monomerinto the charge prior to copolymerization of the electron acceptorfunctional monomer with the electron donor functional monomer. Thisthird monomer is preferably electronically inert, that is substantiallyincapable of forming a charge transfer complex with either thestructural unit having pendant therefrom the strong electron donormaterial or the structural unit having a pendant therefrom the strongelectron acceptor. By thus increasing the spacing between suchelectronically active structural units, the frequency of charge transferinteraction is reduced and consequently the color of the polymericmaterial. Generally, anywhere from 5 to 30 mole percent ofelectronically inert monomer can be added to the charge containing theelectronically active monomers. Although the extent of charge transferinteraction within this diluted copolymer is reduced, it still retainsits ability to effectively transport both holes and electrons, which areinjected into it, irrespective of the source of charge carriers. Thisdiluted copolymer can, therefore, be used as a charge transport matrix.This matrix can be used as a binder for other photoconductive materialsor in composite photoconductive films wherein one layer of the compositeis primarily responsible for photogeneration of charge carriers and thesecond layer of the composite is primarily responsible for transport ofthe photogenerated charge carriers which are injected into it from thephotoresponsive layer contiguous therewith. Due to the presence of bothelectron donor and electron acceptor groups in this copolymer, thismaterial is suitable for use as an ambipolar charge transport matrix.The efficiency with which an ambipolar matrix transports both species ofcharge carriers is, of course, dependent upon the proximity of matchingof the energy levels of (a) the electronic vacancy created uponphotoexcitation of the photoconductive material with the energy level ofthe hole transport level associated with the donor groups of thepolymeric composition; and (b) the energy level occupied by theelectrons generated upon photoexcitation of the photoconductivematerials with the energy level of the electron transport levelassociated with the acceptor group of the polymeric composition. Theefficient transport of both species of charge carriers within such amatrix (especially when the photoconductive pigment is dispersed in thematrix) presumes that the materials dispersed in the matrix do notthemselves trap either species of charge carrier.

The Examples which follow further define, describe and illustratepreparation and use of the copolymers of this invention. Techniques andequipment in the preparation and evaluation of such copolymers arestandard or as hereinbefore described. Parts and percentages appearingin such examples are by weight unless otherwise stipulated.

EXAMPLE I Preparation of poly(1-(2-anthryl)-ethyl methacrylate)

About 0.84 moles (150 grams) of anthracene is dispersed in 150milliliters nitrobenzene. This dispersion is prepared in a reactionvessel equipped with an addition funnel, a thermometer, a source ofnitrogen gas and a magnetic stirring bar. The dispersion is chilled toabout 15° C. In a separate container about 1.9 moles (255 grams) ofaluminum chloride is dissolved in 480 milliliters nitrobenzene. About1.6 moles (155 milliliters) acetic anhydride is added to the aluminumchloride solution by dropwise addition. The aluminum chloride solutionis rapidly agitated during such addition. The temperature of thissolution is carefully monitored since the formation of the complexbetween the aluminum chloride and the acetic anhydride is stronglyexothermic. Subsequent to formation of this complex, it is transferredto the addition funnel. The reaction vessel containing the anthracenedispersion is purged of air with nitrogen, the anthracene dispersionvigorously agitated and the aluminum chloride/acetic anhydride complexadded dropwise over a period of about 60 minutes. The temperature of theanthracene dispersion is maintained at 15° C. during the addition ofthis complex. About 5 hours after completion of addition of the complexto the anthracene dispersion, the reaction of these materials isquenched by the addition of 1500 milliliters of cold, dry benzene(cooled to ˜8° C.). The reaction vessel is chilled in an ice bath afterthe addition of benzene and maintained at this temperature forapproximately 4 hours. The red solids formed during this reaction areseparated from the reaction mass by filtration, washed with additionalamounts of dry benzene and hexane for removal of nitrobenzene residuesfrom the solid. The filtration and subsequent washing of the recoveredsolid should be preformed in a low humidity environment in order toprevent premature hydrolysis of the recovered solid product. Subsequentto removal of residual traces of nitrobenzene from this product, it ishydrolyzed in an aqueous solution of hydrochloric acid (200 millilitersof concentrated HCl per 2 liters distilled water). The solids are thenrecovered by filtration, washed continuously with distilled water untilall traces of acidity are removed, dried in a vacuum oven and purifiedby recrystallization from a benzene/hexane (1:1) solvent mixture. Therecovered product, 2-acetyl anthracene, is light-green in appearance.Yield: 98 grams, M.P. 188° C.

About 53 grams of 2-acetyl anthracene is dispersed in 1,800 millilitersof ethanol, the dispersion heated to boiling under reflux conditions and25 grams of sodium borohydride in 280 milliliters distilled water addedby dropwise addition. During the addition of the sodium borohydride, thedispersion is maintained in a constant state of mild agitation. With theaddition of about two-thirds of the sodium borohydride solution, thedispersed matter dissolves in the solvent and turns brown in color. Uponcompletion of additon of the sodium borohydride, the resulting solutionis heated under reflux conditions for an additional two hours. At thistime, the reflux condensor is opened and approximately 2/3 of thevolatile solvent contained within the mixture allowed to escape. Theproduct remaining in the reaction vessel is isolated from excess sodiumborohydride by hydrolysis with an aqueous solution of hydrochloric acid(200 milliliters HCl per two liters of distilled water). Uponprecipitation of the isolated product, it is filtered, washed withalternate solutions of aqueous hydrochloric acid and distilled water,dried and recrystallized from benzene. The recovered product,1-(2-anthryl) ethanol, is white in color. Yield: 50 grams, M. P. 164' C.

About 50 grams of 1-(2-anthryl) ethanol is dissolved in 375 millilitersof dioxane. To this solution is subsequently added 37.5 milliliters oftriethylamine and 27.5 milliliters methacryloyl chloride. Thecondensation of the 1-(2-anthryl) ethanol and methacryloyl chloride isallowed to proceed for about 24 hours. After that time, the reactionbetween the 1-(2-anthryl) ethanol and methacryloyl chloride is quenchedby the addition of water to the reaction medium. Sufficient water isadded to extract unreacted methacryloyl chloride from the reaction mass.The precipitate which forms is separated from the reaction medium byfiltration, dried in a vacuum oven and recrystallized from a mixedsolvent of benzene and methanol.

EXAMPLES II - VII

The monomer synthesis of Example I is repeated except for thesubstitution of the following acylating agents for acetic anhydride.

    ______________________________________                                        Example No.     Acylating Agent                                               ______________________________________                                        II              formyl chloride                                               III             acetyl chloride                                               IV              propionyl chloride                                            V               butyryl chloride                                              VI              valeryl chloride                                              VII             caproyl chloride                                              ______________________________________                                    

EXAMPLE VIII Synthesis of 2,4,7-trinitro-9-fluorenyl methacrylate

2,4,7-trinitro-9-fluorenone is contacted with stoichiometric quantitiesof hydrazine in an appropriate solvent, heated to a temperature in arange of from about 80° to about 85° C. and the materials allowed toreact for a period of at least 2 hours. At the end of this interval, thedesired product forms a precipitate which can be separated from theunreacted materials and solvent vehicle by simple filtration techniques.The product which is recovered can be subsequently purified by washingin an appropriate solvent. Chemical analysis of the product confirms itto be the 9-hydrazone derivative of 2,4,7-trinitro-9-fluorenone.Following air drying, this 9-hydrazone derivative is placed in areaction vessel with a stoichiometric excess of argentous oxide, andsufficient tetrahydrofuran added to dissolve these materials. Thesolution is then heated to boiling under reflux conditions for a periodof about 5 hours. The precipitate which forms during this reaction isseparated from the solution by filtration and discarded. The liquid ofthe solution is then evaporated sufficiently until crystallization of9-diazo product occurs. Such crystals are separated from the liquid byfiltration and purified by recrystallization from nitromethane. Chemicalanalysis of the purified product indicates it to be the 9-diazoderivative of 2,4,7-trinitro-9-fluorenone.

About 10 parts of this 9-diazo derivative is suspended in 150milliliters of tetrahydrofuran and methacrylic acid added, the quantityof methacrylic acid being 7 fold the stoichiometric amounts required foresterification with the 9-diazo compound. Subsequent to addition of themethacrylic acid to the reaction medium, boron trifluoride etherate isadded to the contents of the reaction vessel. The gram equivalentmolecular weight of the boron trifluoride etherate is equivalent to thegram equivalent molecular weight of the 9-diazo compound. The contentsof the reaction vessel are allowed to react at room temperature for aperiod of about 1 hour. The liquid phase of the reaction mass is removedby rotary evaporation at 50° C. The yellowish solid which is recoveredis subsequently washed in 150 milliliters of water with vigorousagitation for period of 2 hours. The solids are then removed from theaqueous solution by filtration, dried, and dissolved in 50 millilitersbenzene. The solution is heated to boiling under reflux conditions, theparticulates which precipitate are removed by filtration and discardedand the remaining liquid evaporated to an oily residue which eventuallycrystallizes. These crystals are purified by washing with ether.Chemical analysis indicates the crystalline product to be2,4,7-trinitro-9-fluoroenyl methacrylate, m.p. 143°-145° C.

EXAMPLE IX

About 2 grams of the monomer of Example I (5 × 10⁻ ³ moles) and 1.5grams of the monomer of Example VIII (5 × 10⁻ ³ moles) are dissolved in20 milliliters of reagent grade acetone. About 0.5 weight percentazobisisobutyronitrole is now added to the monomer charge, the tubecontaining the monomer charge taken through two freeze/thaw cycles, thetube sealed, and polymerization carried out on a hot water bath at 60°C. for 18 hours. At the end of this interval, the tube's contents areemptied into methanol, the polymer which precipitates separated byfiltration, and purified by reprecipitation from tetrahydrofuran intohexane. Yield about 2 grams ofpoly[1-2(anthryl)-ethyl-methacrylate-co-2,4,7-trinitro-9-fluorenylmethacrylate].

About 0.5 grams of the copolymer is dissolved in 5 milliliters oftetrahydrofuran and the resulting solution draw bar coated on aball-grained aluminum substrate. The copolymer coating is allowed to dryin a vacuum until substantially free of solvent residues. Theelectrophotographic response of this copolymer film is then determinedby conventional techniques using both positive and negative chargingtechniques. In each instance, the electrophotographic response of thefilm is satisfactory and the images produced during such evaluation ofacceptable quality.

EXAMPLE X

The procedures of Example IX are repeated except for the addition of 5 ×10⁻ ³ moles methylmethacrylate to the monomer charge prior to initiationof copolymerization. The resulting product obtained from this synthesisis less highly colored than the copolymer product obtained in ExampleIX. About 0.5 grams of the terpolymer prepared as described above isdissolved in 5 milliliters of tetrahydrofuran and the resulting solutiondraw bar coated over a thin film of amorphous selenium which has beenvacuum deposited upon a ball-grained aluminum plate. The thickness ofthe amorphous selenium layer is approximately 1 micron and the dry filmthickness of the terpolymer coating approximately 5 microns. Theterpolymer coating appears only slightly colored in contrast to thecopolymer coating prepared by the procedures described in Example IX.The surface of the terpolymer coating is charged to a positive potentialand image information projected onto its surface with light that hasbeen filtered to prevent photoactivation of the terpolymer layer. Thelatent image pattern thus produced is rendered visible by developmentwith polar liquid developer and thereafter transferred to a supportivesubstrate. The developer residue remaining on the surface of the polymerfilm are removed and the imaging process repeated except for thereversal in the polarity of the sensitizing charge and the reversal inthe polarity of the charge on the developer materials. In bothinstances, copy quality is acceptable and reproducible.

What is claimed is:
 1. A random copolymer of the formula: l ##STR5##wherein R is hydrogen or methyl;R' is hydrogen or methyl; R" is alkyl of1-5 carbon atoms; R'" is selected from the group consisting of --NO₂,halogen, --CN and CF₃ ; X and Y are independently selected from thegroup consisting of hydrogen, chlorine, bromine, alkyl of 1- 4 carbonatoms and phenyl; a and b can range from 0-4; and n can range from about5 to about 95 percent of the total number of structural units of thecopolymer; and m can range from about 5 to about 95 percent of the totalnumber of structural units of the copolymer.
 2. A method for performingfree radical initiated copolymerization of an addition monomer havingpendant therefrom a strong electronic donor group with an additionmonomer having pendant therefrom a strong electronic acceptor group,said method comprising contacting the following alkylacrylate monomerswith one another in the presence of a free radical initiator: ##STR6##wherein R is hydrogen or methyl;R' is hydrogen or methyl; R" is alkyl of1- 5 carbon atoms; R'" is selected from the group consisting of --NO₂,halogen, --CN and --CF₃ ; X and Y are independently selected from thegroup consisting of hydrogen, chlorine, bromine, alkyl of 1-4 carbonatoms and phenyl; a and b can range from 0-4; and n can range from about5 to about 95 percent of the total number of structural units of thecopolymer; and m can range from about 5 to about 95 percent of the totalnumber of structural units of the copolymer.